Ceramic resistor



Patented Jan. 2, 1951 CERAMIC RESISTOR Francis X. Sorg, St. Marys, Pa., assignor to Stackpole Carbon Company, St. Marys, Pa., a corporation of Pennsylvania No Drawing. Application May 24, 1949,

Serial No. 95,163

14 Claims. (Cl. 20175) This invention relates to ceramic resistors, and more particularly to ceramic resistors used at elevated temperatures, for example as suppressors in spark plugs.

A primary object of the invention is to provide ceramic resistors that are of simple composition, that combine good mechanical strength and requisite electrical resistivity with the ability to be operated at elevated temperatures without objectionable impairment of those properties, and that may be made easily from readily available materials.

Another object is to provide such resistors that additionally are adapted to carry repeated electrical surges at temperatures up to about 500 F. with minimum change in resistance. I

A further object is to provide a method of making ceramic resistors embodying the foregoing properties and characteristics and which is simple and easily practiced using conventional equipment.

Oth r objects will appear from the following description.

The invention is predicated u on my discovery that resistors in accordance with its objects are provided by vitrified refractory bodies formed from clay, talc and zircon through which carbon in finely d vided form is uniformly distributed to permit electrical leakage to the extent necessary to supply the desired resistance. More particularly, the bodies of resistors in accordance with the invention are made from ceramic compositions comprising about 8 to 15 per cent of talc, about to per cent of clay, most suitably a ball clay, and about 40 to 5''! per cent of z rcon, a l by wei ht. Preferably the bodies are formed from, by weight, about 10 per cent of ta c, 40 per cent of ball clay, and per cent of zircon.

Such compositions when vitrified provide ceramic bodies that are mechanically strong and of high electrical resistance and, imnortantlv, they are stable mechanically and electrically when exposed to el vated temperatures, for instance as high a 500 F. Compositions of clay and talc do not suffice for these purposes because they are mechanicallv weak and their electrical properties are insufficient for resistor purposes: furthermore. they do not satisfactorily withstand use at elevated temperatures or the ap lication of re eated electrical sur es. A thou h zircon is hi hly refractorv and has very hi h d electric str n th. it can not be u ed alone for makin resister bod es because it cannot be vitrifi d at anv practicable temperature. Nor do compositions of zircon and talc or zircon and clay provide satisfactory resistor bodies. I have discovered, however, that the clay and talc react to vitrify readily and serve as a permanent bond for zircon, and that the product forms resistor bodies of adequate mechanical strength, the necessary electrical resistance for resistor purposes, and retention of those properties at elevated temperatures. Not only does the zircon contribute high dielectric strength to the bodies thus produced, but also it reduces the porosity of claytalc compositions alone.

In accordance with the invention carbon in finely divided form is distributed throughout the resistor body. The amount used will, of course, depend upon the specific resistance desired, i. e.. the more carbon that is present, the lower the resistance will be. For most purposes I prefer to add to the ceramic body composition from one to ten parts by weight of finely divided carbon per weight parts of the ceramic body constituent. Preferably there is used for this purpose a calcined' carbon black. The following table shows how the resistance may be varied by alteration in the-amount of carbon added to a body of the foregoing preferred composition.

Parts by weight car- Spec. Res. bon per 100- Ohms/In.

parts body Cube mix All of the materials used should be in finely divided form. The zircon is preferably supplied in initially milled condition, and it is important that the zircon be in the form of rounded particles because if the zircon is in the form of particles with sharp points there is a tendency to cause concentration of electric field stress that is undesirable in a resistor. In making resistors in accordance with the invention the ceramic constituents and carbon are intimately mixed so that there is homogenous distribution of all of them throughout the batch. Most suitably this is accomplished by ball milling, and preferably the mixture is ground so that the particles do not exceed about 10-micron size. A temporary binder is then added, suitably in a, kneading type mixer. Preferably the binder is one that will be completely burned out during the firing of the resistor bodies. A wide variety of such binders are known andused in the ceramic trade such, for example, as molasses, lignin residue from the wood pulping industry, and others, such as diglycol stearate. Likewise, a mixture of tricresyl phosphate with a silicone resin, such as DC 2103 resin" has been found to give satisfactory results because it affords adequate temporary bondin and decomposes during firing without depositing any appreciable carbonaceous residue that might alter the electrical characteristics desired.

After the batch has been thoroughly mixed with the temporary bnder the material is granulated through a 20-mesh screen and then molded to size and Sh&fl9. ,Aftei' mo ding the bodies are fired between 2000 and 3000 F., dependin upon the composition of the body. in a non-oxidizing atmosphere. preferab'y in a reducing atmosphere such as hydro en or cracked ammonium.

According to the provis ons of t e patent statute I have exnlain d the principle and mode of practicing mv invention and have described what I now consider to represent its best embodim nt. However, I desire to have it understood that, with n the scone of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. That met od of making ceramic re istors that COlIlDliFBq t e steps of formin a batch con sisting ess ntially. by wei ht. of 8 to 15 per cent of finely divided talc. 35 to 45 per cent of fin ly divid d cl y. and 40 to 57 per cent of milled zircon in th form of finely divid d ro nded artic es. together with 1 to 10 parts by wei ht of finelv divid d carbon ba ed on the wei ht of said mxt re. mix ng said batch with a temporary binder. then molding re istors therefrom. and heatin t e molde resistors in a non-oxidizing atmos h re to vitrify them.

2. That met od of making ceramic resistors that comprises he steps of formin a batch consisting essentially, by weight, of 8 to per cent of talc, 85 to 45 per cent of clay, and 40 to 57 per cent of milled zircon. all in the form of particles not over about 10 micron size, together with l to 10 parts by weight of finely divided carbon based on the weight of said mixture, mixing said batch with a temporary binder, then molding resistors therefrom, and heating the molded resistors in a non-oxidizing atmosphere to vitrify them. I

3. A method according to claim 2, said carbon being calcined carbon black.

4. A method according to claim 2, said zircon being in the form of rounded particles.

5. A method according to claim 2, said zircon being in the form of rounded particles, and said carbon being calcined carbon black.

6. A method according to claim 2, said atmosphere being a reducing atmosphere.

7. A method according to claim 2, said zircon being in the form of rounded particles, and said atmosphere being a reducing atmosphere.

8. That method of making ceramic resistors that comprises the steps of forming a batch consisting essentially, by weight, of 10 per cent of finely divided talc, 40 per cent of finely divided ball clay, and 50 per cent of milled zircon in the form 'of finely divided rounded particles, together,

with 1 to 10 parts by weight of finely divided carbon black based on the weight of said mixture. mixing said batch with a temporary binder, then molding resistors therefrom, and heating the molded resistors in a non-oxidizin atmosphere at 2000 to 3000 F. to vitrify them.

- 9. A method according to claim 8, said atmosphere being reducing.

10. A method according to claim 8, said talc, ball clay and zircon being not over about 10 micron size.

11. A method according to claim 8, said talc, ball clay and zircon being not over about 10 micron size, and said atmosphere bein reducing.

12. Ceramic resistors consisting essentially of a vitrified body of a mixture of, by weight, 8 to 15 per cent of talc, 35 to per cent of clay, and 40 to 57 per cent of milled zircon, together with 1 to 10 parts by weight of finely divided carbon based on the weight of said mixture.

13. Ceramic resistors consisting essentially of a vitrified body consisting essentially, by weight, of a mixture of 10 per cent of finely divided talc, 40 per cent of finely divided clay, and per cent of milled zircon, together with 1 to 10parts by weight oi. finely divided carbon based on the weight of said mixture.

14. Ceramic resistors consisting essentially of a vitrified body 01' an intimate mixture, by weight, of 10 per cent of talc, 40 per cent of ball clay, and 50 per cent 01 milled zircon in the form of rounded particles. all in the form of particles not over about 10 micron size, together with 1 to 10 parts by weight of finely divided carbon black based on the weight of said mixture.

- FRANCIS X. SORG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Buckman Jan. 6, 1925 Harrington Oct. 21, 1930 Bradley Feb. 16, 1932 Benkelman Oct. 3, 1933 Grisdale Oct. 14, 1941 Herrick et a1 May 12, 1942 Number 

1. THAT METHOD OF MAKING CERAMIC RESDISTORS THAT COMPRISES THE STEPS OF FORMING A BATCH CONSISTING ESSENTIALLY, BY WEIGHT OF 8 TO 15 PER CENT OF FINELY DIVIDED TALC, 35 TO 45 PER CENT DIVIDED CLAY, AND 40 TO 57 PER CENT OF MILLED ZIRCON IN THE FORM OF FINELY DIVIDED ROUNDED PARTICLES, TOGETHER WITH 1 TO 10 PARTS BY WEIGHT OF FINELY, DIVIDED CARBON BASED ON THE WEIGHT OF SAID MIXTURE, MIXING SAID BATCH WITH A TEMPORARY BINDER, THEN MOLDING RESISTORS THEREFROM, AND HEATING THE MOLDED RESISTORS IN A NON-OXIDIZING ATMOSPHERE TO VITRIFY THEM. 